Usually you take full BGP routing tables when you have multiple ISP links at a single site, either to load-balance outbound traffic between ISPs based on which ISP has a better route to a given destination, or to provide failover at the prefix level rather than the link level. (If ISP-A loses its route to a given network, it will withdraw that route from your BGP table so that you know to send traffic destined to that network to the other ISP. When using only a default route to each ISP, your router will never know about such a failure -- it only knows when the default route is withdrawn by the ISP or, more often, when the link itself goes down.)
So, given that you only have a single router/ISP link at each site, you probably don't need to accept full BGP tables. But it depends on what (if any) failover/load-balancing stuff you're doing. I see now, however, that 2691s can handle 256MB of memory (128MB is the limit for the rest of the 2600 series), so taking a full BGP table from each ISP shouldn't be a problem actually.
As for 802.1q trunking, generally you need an IP Plus version of IOS to do this, which is a step up from the standard IP builds. You can use Feature Navigator (http://www.cisco.com/cgi-bin/Support/FeatureNav/FN.pl) to confirm.
